A signal processing arrangement in the form of a telephone speech circuit is connected to a subscriber line and performs several functions. In addition to processing speech signals fed from a subscribers telephone handset the speech circuit acts as a regulator.
For example, the subscribers line carries both a D.C. voltage and an A.C. speech signal and is connected to the speech circuit via a diode bridge. The diode bridge provides an A.C. speech signal superimposed on a D.C. level.
The subscribers line may be either relatively long, having a relatively high impedance or may be relatively short and have a comparatively low impedance. In the former case, there is relatively little D.C. current available and the speech circuit must draw maximum available current and have only a small voltage drop across itself. The speech circuit therefore acts as a voltage regulator so that the voltage across itself will remain almost constant and low.
When the line is shorter the current drawn from the line is limited by the speech circuit itself which acts as a current regulator.
In practice, the voltage supplied by the diode bridge is compared with a reference potential and a D.C. control signal is derived from this comparison and utilized to control the current drawn by the speech circuit.
The D.C. control signal will inevitably contain A.C. components which would upset the regulation functions and therefore these components should ideally be removed. A filter may be used in the output of the comparator but inevitably second order components of unwanted A.C. signals would be present and these preferably should be removed. Second order filtering therefore forms part of the speech circuit and involves the use of a capacitance.
Other functions of the circuit also involve the use of capacitances and these include A.C. coupling for speech signals and impedance matching of the circuit to the subscriber line.
It is desirable to produce the circuit as an integrated circuit and the use of a large number of capacitors to achieve the above functions is undesirable because this would increase the cost of the circuit. Further it is also desirable to keep the number of pins required on the integrated circuit package to a minimum and this requirement also makes it desirable to keep the numbers of capacitors required in the circuit to a minimum.